a) Field of the Invention
This invention relates to a novel PVA-base copolymer and also to its production process and use. More specifically, the present invention is concerned with a novel PVA-base copolymer comprising a PVA chain and an aliphatic polyester grafted as a ring-opened polymer (aliphatic polyester) of said lactone on said PVA chain and useful as films having high flexibility and excellent waterproofness, formability, gas barrier properties and safety, hot-melt adhesives, release treatments, various coating materials suited for molded or otherwise formed products of vinyl chloride resins and ink jet recording sheets, binders for paints, inks and the like, packaging materials, forming materials and the like; and also with its production process and use.
b) Description of the Related Art
PVA has excellent properties such as water solubility, solvent resistance, adhesiveness, film formability, gas barrier properties and safety and has conventionally been used in a wide variety of fields as forming materials for fibers, paper, adhesives and films.
For its water solubility, PVA is however poor in waterproofness. Further, PVA coatings lack flexibility although they are strong and tough. To overcome this problem, PVA coatings are used by imparting flexibility thereto with water or an alcoholic plasticizer. These coatings are however accompanied by the drawback that they have high humidity-dependency and cannot exhibit stable performance.
In addition, PVA undergoes gradual thermal decomposition when heated to temperatures of 100.degree. C. or higher in air. For its high softening point, it has heretofore been unable to obtain films, sheets or like formed products from PVA by any process other than the solution process (dissolution process) when PVA is used in a water-free or unplasticized form.
To improve the formability of PVA, PVA is converted into derivatives such as polyvinyl formals and polyvinyl butyral by chemical reactions, and these derivatives are then used as materials for various films, sheets and other formed products.
The modification into such a formal or butyral form, however, unavoidably leads to reductions in properties inherent to PVA, such as water solubility, gas barrier properties and biodegradability.
As a method for imparting flexibility to PVA without relying upon such a chemical modification, there is a method that plasticizes PVA with a plasticizer such as glycerin. However, the plasticizer incorporated in PVA is extracted out with a solvent or bleeds out to a surface of a formed product along the passage of time. Further, because the plasticizer is caused to evaporate significantly upon its melt forming, PVA obtained by this method involves the problem that this evaporation of plasticizer may deteriorate the quality and workability of the resulting formed product and may also contaminate the working environment or the like.
With a view to overcoming the above-mentioned problems of a plasticizer in PVA, a variety of investigations have been conducted. For example, Japanese Patent Application Laid-Open (Kokai) No. SHO 51-123257 discloses use of an esterified product between a polyhydric alcohol and a polycarboxylic acid as a plasticizer, and Japanese Patent Application Laid-Open (Kokai) No. SHO 49-120946 discloses use of an esterified product between glycerin and phthalic acid as a plasticizer. The use of these plasticizers, however, has not brought about any satisfactory results yet.
Also reflecting the environmental problems and the like in recent years, there is an outstanding desire for the development of a high-molecular material which has water solubility and safety and enables direct manufacture of sheets, films and various other formed products by various forming machines without using the dissolution process or a plasticizer.